Guide

Vision system PLC integration: trigger, result, tracking and the reject

Trigger, result handover, tracking, reject confirmation and fault handling, for automation and controls engineers.

By Frank GuoTechnical review: Frank GuoPublished 24 September 2026Updated 27 October 202611 min read

Applies to:Presence & counting Fill level, cap & seal Carton & pallet

// Short answer

Vision system PLC integration connects the camera station to the line so that failed products are removed. A sensor triggers the camera when a product arrives, the station decides pass or fail, and the result goes to the line PLC over digital I/O or an industrial network, depending on the camera or controller model. The PLC tracks the failed product to the reject, by time delay on constant-speed lines or by encoder counts where speed varies, fires the reject and, with a confirmation sensor, checks the product left. Fault behaviour is agreed in the design.

Key points

The camera decides pass or fail; the PLC tracks the product and fires the reject.
Digital I/O is simple and direct; an industrial network carries more data. Which protocols are available depends on the camera or controller model.
A fixed time delay suits constant-speed lines; an encoder follows conveyor movement when speed changes, as long as products do not slip or accumulate on the belt.
A reject is only finished when a sensor confirms the product left the line.
Behaviour on faults is designed, written down and tested. It is general control practice, not a certified safety function.

The signal chain at a glance

A vision station does not reject anything by itself. It decides whether a product passes; the line's PLC decides what happens next. Between the two sits a chain of signals: a trigger, an exposure, a decision, a result, tracking, the reject and a confirmation. Each link can fail, and most problems on real lines come from the links around the camera rather than from the inspection itself. The inspection technology overview shows how the parts of a station fit together.

The diagram shows the order of events for one failed product. The gaps between them depend on line speed, processing time and the distance from camera to reject, so they are shown without values.

[Schematic]Timing diagram · illustrative · not to scale

Swipe sideways to see the whole drawing →

[1] Trigger: a photo-sensor or encoder position signals that a product is in view.

[2] Exposure: the camera captures the image, with the light strobed or on.

[3] Decision: the inspection runs on the recipe for the current product.

[4] Result: pass/fail with a "result valid" signal, over I/O or network.

[5] Tracking: the PLC holds the result until the product reaches the reject.

[6] Reject: the air jet, pusher or diverter fires on the failed product.

[7] Confirmation: a sensor checks the product left the line within the window.

Trigger: telling the camera a product has arrived

Most stations use a photo-sensor mounted just before the camera. When the product breaks the beam, a hardware trigger goes to the camera or vision controller, and the light is strobed or switched on for the exposure. The sensor must see every product once and only once: gaps between products, transparent containers and wobbling products are the usual causes of missed or double triggers.

An encoder on the conveyor gives position instead of time. It is used when speed varies, when products are close together, or when a line-scan camera builds the image line by line as the product moves. The same encoder is often used for reject tracking.

A short exposure keeps a moving product sharp; how short depends on speed and the smallest detail to be seen, and is set during the sample test.

Handing over the result: digital I/O or network

Digital I/O is the simplest route: the station switches outputs wired to PLC inputs, typically pass/fail, a "result valid" pulse that tells the PLC when to read it, ready, and alarm. It is fast, easy to test with a meter and independent of network settings. Output type (NPN or PNP) must match the PLC input card.

An industrial network carries more: the codes read, measurements, the active recipe, counters and a sequence number for each product. It needs the right protocol on both sides.

Protocol support depends on the camera or controller model, not on the software brand as a whole. For the platform we use, HIKROBOT's catalogue states that VisionMaster supports TCP/IP, Modbus, serial port, UDP, EtherNet/IP and other common industrial protocols. Its smart code readers list PLC protocols per model; the IDH7000 row, for example, lists SmartSDK, TCP, FTP, PROFINET, EtherNet/IP, Modbus, UDP and serial, while other reader rows list MELSEC/SLMP and Fins (HIKROBOT Machine Vision Smart Product Catalog, 2025 Q4). PROFINET is therefore listed on specific devices, not everywhere. Plants also use EtherCAT, or OPC UA to pass data to MES systems; each needs support on the chosen device or a gateway. We check the exact model against your PLC before quoting; where the protocol does not match, a gateway or hard-wired I/O is an option. addaScan builds stations on HIKROBOT VisionMaster; this website does not claim any official HIKROBOT partnership or distributor status.

Tracking the failed product to the reject

The camera and the reject are usually some distance apart, and other products pass in between. The PLC must fire the reject on the right product.

Time delay. The PLC starts a timer when the fail result arrives and fires the reject when it expires. This works on constant-speed lines where products do not slip. It goes wrong when the line slows, stops and restarts, or when products move at different speeds.

Encoder-based tracking. The PLC records the encoder position of each inspected product in a shift register (a queue of positions and results). When the conveyor has moved the product to the reject, the PLC fires. Because tracking follows distance, not time, it stays correct through speed changes and stops, provided each product moves with the belt. The encoder measures the belt, not the product: where products slip, accumulate or are pushed together, add a sensor at the reject point to re-identify the product. A time delay must also allow for the variation in processing and actuator time, not only the average.

Two checks keep tracking honest: count triggers against results, so a missing result is noticed; and keep products from accumulating or changing order between camera and reject.

Reject devices: pusher, air jet, kick-out, diverter, line stop

Rejection systems come in several forms. The reject system, whether a bottle reject system on a filling line or a conveyor reject system on a packing line, is chosen for the product, the gap between products and the line speed, not for the camera.

Air jet (blow-off): a timed blast of compressed air. Suits light products such as empty bottles, cartons, sachets and labels at high speed. Needs clean, dry air at stable pressure, a pressure switch, and a nozzle aimed so that only the failed product moves.
Pusher (kicker, kick-out): a pneumatic or electric arm pushes the product off the conveyor. Suits heavier or upright containers. Its stroke time limits line speed, and tall glass containers can topple, so the push is shaped and tested on your product.
Diverter or gate: swings or slides the product into a side lane. Gentler, for heavy or fragile products and for products that must be recovered; slower to switch.
Drop-down flap: the conveyor opens and the product falls into a bin. Used for pouches, bags and small packs.
Line stop: where a product cannot be removed automatically, such as a continuous web or a heavy pallet, the line stops or slows and an operator removes it; web lines mark or map the defect instead.

A lockable reject bin with a bin-full sensor keeps rejected product away from good product, which food safety audits look at closely. Operators on lines without a working reject system often describe the problem as “how do we track the bottles to be ejected?”; the answer is the tracking described above plus one of these devices.

Confirming the reject

Firing the reject is not proof the product left. A confirmation sensor at the chute, in the bin, or just after the reject on the conveyor checks that the failed product left within an agreed window. If the product is still on the line, or a good product was removed, the PLC raises an alarm or stops the line. A bin-full sensor and an air-pressure switch complete the picture.

Designing for faults

What the line does when something goes wrong is a design decision, agreed with your production and quality teams and tested at FAT and SAT. The questions we work through include:

No result in time: is the product rejected, or the line stopped?
Camera or controller offline: how does the PLC know (heartbeat, ready signal), and does it stop the line, reject everything, or alarm?
Air pressure low: the reject may not work; should the line keep running?
Confirmation failed or bin full: alarm, stop, or both?
Trigger without a product, or a product without a trigger: how is each detected?
Inspection bypass: who may switch it off, how is it shown and how is it logged?

This is general control practice. It is not a certified machine-safety function; where a fault could harm people, that is handled by the machine's safety system and its risk assessment.

Power interruptions and restarts

Power interruptions, including load shedding and short dips, stop a line in an uncontrolled way. The camera, the PLC and the reject may come back at different times, and whatever was between the camera and the reject is no longer tracked. This is how we design stations to handle it; the exact behaviour is agreed per line and tested at SAT.

Treat the tracking window as unknown. Products between the camera and the reject when power was lost have no trustworthy result. On restart they are rejected automatically, or the operator clears that section before the line is released.
No product passes uninspected. Until the camera or vision controller reports ready, the PLC holds the line or rejects every product that passes the camera.
Controlled shutdown. A UPS on the vision PC or controller, and where agreed on the PLC, lets them shut down cleanly, so recipes, settings and stored images survive.
Check before releasing production. The station reports its active recipe and the PLC compares it with the product on the line; the air pressure switch must show pressure before the reject is trusted; a short challenge with test packs proves detection and reject again.
Record the interruption. The time of the stop, the counters at that moment and the restart checks are logged, so the batch record shows what happened.

Challenge tests for BRCGS and FSSC audits

Food safety schemes expect online inspection equipment to be tested, not only installed. BRCGS guidance on clause 6.2.4 (Issue 8 FAQ) describes challenging online label and code verification equipment with incorrect packs and testing the detection, the reject or line stop, the authorised restart and the mis-read or no-read alarm. FSSC 22000 version 6 asks for calibration or verification of equipment used for quantity control (2.5.9(b)). Neither requires a camera; both expect proof that whatever checks you use actually works.

We build these tests into the station: a set of marked test packs for each product, a test procedure that runs them at line speed, a no-read test, a reject confirmation test and a record of each test with date and result. How often the tests run is set by your own risk assessment.

Selecting the recipe from the PLC

Each product has a recipe: its references, limits and expected codes. At changeover the PLC sends the recipe number or SKU, the station switches and reports back the recipe it is running, and the PLC checks they match before production restarts. Selection on the HMI also works, with access control. A code read from the product can cross-check that the right product is on the line. The envelope verification case shows read data matched against a batch file before each item is sorted or rejected.

Interface checklist

We agree these items in writing before the design is finished. See how we deliver a project for how FAT and SAT are run.

Vision station to PLC interface checklist
Item	What to agree	Confirmed during
Trigger source	Photo-sensor or encoder position; where it is mounted; one trigger per product.	Site survey
Encoder	Whether one is fitted or needed, and who reads it: camera, PLC or both.	Site survey
Result signals	Pass/fail, result valid, ready, alarm; output type matched to the PLC inputs.	Design review
Network	PLC make and model, protocol, and whether the chosen camera or controller lists it.	Design review
Heartbeat	How the PLC detects that the station has stopped responding.	Design review
Tracking	Time delay or encoder-based shift register; distance from camera to reject.	Design review
Reject actuator	Air jet, pusher or diverter; air supply and pressure monitoring.	Site survey
Confirmation	Sensor position, confirmation window, bin-full detection.	Design review
Fault responses	Agreed action for no result, station offline, low air, failed confirmation, bin full.	FAT and SAT
Recipe selection	How the PLC or HMI selects the product, and how the station confirms it.	FAT
Data	Which results, images and counters are kept, where, and for how long.	Design review
Responsibilities	Who changes the PLC program, who wires the panel, who signs off each test.	Quotation

// Production checks

Two quality-control sequences: cell hold and conveyor rejection

A machining cell normally controls release at a fixture, while a continuous packaging line tracks the item to a downstream reject. Both need a current cycle identity, a valid result and a verified physical outcome.

Decision or event / Check or action / Acceptance boundary
Decision or event	Check or action	Acceptance boundary
No result or timeout	Hold the cell or use the agreed conveyor exception route	Never substitute the last good result.
No-read or absent expected data	Record the reason and prevent unverified release	Do not change the expected recipe from the observed item.
Low air or reject unavailable	Inhibit release or stop/hold according to the site design	A reject command alone is not evidence that the item moved.
Reject bin full or failed confirmation	Apply the defined stop/hold and material quarantine	Identify the affected interval and do not continue uncontrolled.
Restart or rework	Clear stale state and reconcile in-flight material	A restarted PLC must not inherit a pass for a different item.

Define signal ownership, busy/ready/trigger/result/acknowledge states and recovery with the line controls owner. These are quality interlocks; machine safety functions require their own risk assessment and compliant design.

Cell feature release · Pack rejection · Diagnose optical and physical faults

// FAQ

Questions about PLC and reject integration

How does a vision system communicate with a PLC?

Either through digital outputs wired to PLC inputs, such as pass/fail, ready and alarm, or over an industrial network that can also carry data like codes read and recipe numbers. The available protocols depend on the camera or controller model, so the interface is confirmed against your PLC before quoting.

How do you track bottles to reject on a conveyor?

This is how you track bottles to reject, or any other product: the PLC stores each result and releases the reject when that product reaches the reject point. On constant-speed lines a time delay can be enough. Where speed varies or the line stops, encoder counts in a shift register follow each product by position rather than by time, provided products move with the belt; where they slip or accumulate, a sensor at the reject point re-identifies the product.

What is a bottle reject system?

The device and logic that take a failed bottle off the conveyor: an air jet, pusher or diverter, released by the PLC when the tracked bottle reaches it, and a confirmation sensor that checks the bottle left the line. Without tracking and confirmation, a bottle reject system can remove the wrong bottle or none at all.

Why does a labeller reject system throw out good bottles?

Usually timing. On a line whose speed changes, a fixed time delay fires the reject early or late, so the good bottle beside the bad one is removed. Tracking with an encoder fixes the travel distance instead of the time. False rejects that come from the inspection itself are covered in the guide to reducing false rejects.

How can a line confirm that a rejected product was removed?

A sensor at the reject chute or bin, or just after the reject point on the conveyor, checks that the failed product left the line within an agreed window. If it did not, the PLC raises an alarm or stops the line, as agreed in the design.

Does VisionMaster support PROFINET?

Not as a general statement. HIKROBOT's 2025 Q4 catalogue lists PROFINET on specific code reader products, and lists TCP/IP, Modbus, serial, UDP and EtherNet/IP for VisionMaster. For a PROFINET line we confirm the route per model, for example a device that lists it, a gateway or hard-wired I/O.

How do you reject product when the vision system is not synchronised with the conveyor?

Tie tracking to conveyor movement instead of time: an encoder feeds a shift register in the PLC, so the reject fires when the product has travelled the set distance. Where products slip or bunch up, add a sensor at the reject point to re-identify the product, and treat any product whose position is uncertain as a reject.

Does a barcode scanner on the packing line need a reject mechanism?

If it is there to stop wrong or unreadable codes reaching the customer, yes. A scanner that only beeps or logs relies on an operator reacting in time. With a reject or line stop, a no-read or wrong code removes the pack automatically, and a confirmation sensor proves it left.

What should a vision station do after a power failure?

Treat products between the camera and the reject as unknown and reject them or clear the section, hold the line until the station reports ready, check the active recipe and air pressure, run a short challenge with test packs, and log the event. UPS support lets the PC or controller shut down cleanly.

Who changes the PLC program?

This is agreed in the scope. Either your controls team or line builder makes the changes to a written signal specification, or it is part of our scope. Either way the signals are tested at FAT and SAT.

Is the reject logic a safety function?

No. Reject tracking and fault handling are general control practice, not a certified machine-safety function. Hazards to people are handled by the machine's safety system and its own risk assessment.